Assuming you don't want anything fancy (like multiple partitions), just boot off the Windows 7 DVD. When it asks you where you want to install Windows 7, go into the advanced setup. Manually delete all the existing partitions on the drive, then tell Windows 7 to install on that drive. It will set everything up on its own.
Please be aware that many of the Windows 7 keys being sold on eBay are TechNet keys. TechNet was a program Microsoft ran for IT support and sales people, which gave keys to all their software so they could try out the programs and help troubleshoot problems at clients. They were not supposed to be resold.
Microsoft shut down the program due to rampant abuse by ebayers selling the keys. While the keys are legit, they are tied to the original TechNet account. If Microsoft detects a key from a TechNet account being used for piracy (i.e. used multiple times), they will invalidate every key on that TechNet account. So if any of the keys sold by the ebayer end up being spread for piracy, you could turn on your computer one day and the Windows 7 key you bought will be invalid.
This happened to a friend of mine who bought a Win 7 Ultimate key off eBay. He never shared his key but it became invalid one day, and because you cannot downgrade Windows versions he had to pay for a full Windows 7 Ultimate key to fix it. You are better off just buying a retail key from a reputable dealer like Newegg in the first place.